A functional adaptive immune system depends on a diverse and self-tolerant population of T lymphocytes that are generated in the thymus and maintained in the peripheral lymphoid organs. Recent studies have defined the cytokine transforming growth factor-beta (TGF-beta) as a critical regulator of thymic T cell development as well as a crucial player in peripheral T cell homeostasis, tolerance to self-antigens, and T cell differentiation during the immune responses. The long-term objective of this proposal is to elucidate the mechanisms by which TGF-beta regulates T cells. T cell defects observed in mice with T cell-specific deletion of Tgfbr2 gene are associated with abnormal expression of receptors for cytokines of the common gamma-chain receptor family including interleukin 7 (IL-7), IL-2, and IL-15. To determine the function of compromised CD127 (IL-7 receptor alpha chain) expression in TGF-beta receptor II-deficient thymocytes and naive T cells, a strain of CD127 transgenic mice will be used. To determine the role of anomalous CD122 (IL-2/15 receptor beta chain) expression in TGF-beta receptor II-deficient effector T cells, a strain of IL-15-deficient mice will be utilized. These mice will be crossed with T cell-specific TGF-beta receptor II-deficient mice, and the correction of T cell defects will be determined. Defects of TGF-beta receptor II-deficient T cells are additionally associated with abnormal expression of Gfi-1, T-bet, and Eomes, transcription factors that regulate CD127 and CD122 expression. The functions of Gfi-1, T-bet, and Eomes in control of CD127 and CD122 expression and TGF- beta receptor II-deficient T cell activity will be addressed using mice that are deficient in these transcription factors. Finally, the role of TGF-beta-activated Smad proteins in Gfi-1, T-bet, and Eomes repression in T cells will be studied. Successful completion of the projects outlined in this proposal will generate mechanistic insights into the crosstalk between TGF-beta and the common gamma-chain cytokines in T cell regulation.